1. Field of the Invention
The present invention relates to a method for controlling an ejector of an injection molding machine, and to an injection molding machine in which such a method is utilized.
2. Description of the Related Art
In a conventional injection molding machine, a resin heated and melted in a heating cylinder is charged into a cavity of a die device in a pressurized state, and then cooled and solidified in the cavity to obtain a molded product.
Such a die device is composed of a stationary die and a movable die, and the movable die is advanced and withdrawn by a die clamping device so that the movable die contacts and separates from the stationary die, thereby performing die closing, die clamping, and die opening. In the die opening step, the movable die is withdrawn while holding the molded product therein, and the molded product is then separated from the movable die by an ejector device. That is, an ejector pin of the ejector device is advanced to eject the molded product from the movable die.
To this end, the ejector pin is disposed with its tip end facing the cavity and with its rear end fixed to an ejector plate, and an ejecting cylinder is connected to the back of the ejector plate through an ejector rod. Thus, when the ejecting cylinder is operated, the ejector plate is advanced via the ejector rod so that the ejector pin fixed to the ejector plate moves forward to eject a molded product.
To obtain a mechanical connection between the ejector cylinder and the ejector plate, the ejector rod is fixed to the ejector plate, or is arranged to contact and separate from the ejector plate. In the latter case, the ejector plate cannot be withdrawn by the ejector rod, and therefore a return spring is provided to withdraw the ejector plate.
FIG. 1 is a diagram showing ejection of a molded product by a conventional ejector device.
In FIG. 1, reference numeral 131 denotes a molded product, reference numeral 132 denotes a movable die, and reference numeral 133 denotes an ejector pin, the rear end of which is fixed to an unillustrated ejector plate. The molded product 131 is ejected by the ejector pin 133.
However, if a large frictional resistance exists between the molded product 131 and the movable die 132 when the ejector pin 133 contacts the molded product by its advancing movement in the advancing step of an ejecting operation, the tip of the ejector pin 133 thrusts into the molded product 131, thereby damaging the molded product 131, or the molded product 131 scatters in all directions upon separation from the movable die 132.
When the molded product 131 is separated from the movable die 132, the molded product 131 is first deformed into an arched shape, as shown by a solid line in FIG. 1, and is then restored from the deformed state to its original state. After that, it is deformed in the opposite direction, as shown by a broken line. Accordingly, the molded products may scatter in all directions by the deformation restoring force, and/or cracks or the like may be produced in the molded product 131.
When the ejector pin is withdrawn in the withdrawing step, the molded product 131 which has been separated from the movable die 132 may be placed back into the cavity of the movable die 132 due to the attachment of the molded product 131 to the ejector pin 133. Also, the molded product 131 may remain in the movable die 132 due to partial engagement therewith or due to static electricity.
To prevent the molded product 131 from being damaged or scattered during the advancing step, an ejector device has been proposed in which the speed of the ejector pin 133 is changed in the middle of the advancing movement (the rightward movement in FIG. 1) from the withdrawn position to the projected position. With this control, the speed of the ejector pin 133 can be lowered before the ejector pin 133 contacts a molded product 131.
However, in the conventional ejector device, the ejection speed cannot be changed at an optimal position, and the ejector pin 133 therefore advances at a low speed even before contacting the molded product. Accordingly, the ejecting operation involves a loss of time, so that a longer time is needed to complete the ejecting operation, and production costs increase accordingly.
In another ejector device, the ejector pin 133 is reciprocated a plurality of times, by an unillustrated ejecting cylinder, with repeated advancing and withdrawing. That is, after an advance from the withdrawn position to the projected position, the ejector pin 133 is withdrawn to a preset intermediate position in the stroke or to the withdrawn position (the original point of the stroke).
The ejector pin 133 is reciprocated a plurality of times between the projected position and the preset position or between the projected position and the withdrawn position.
In the latter systems, at least 0.1 seconds is needed to withdraw the ejector pin 133 to the preset position or the withdrawn position. Accordingly, if the ejector pin 133 is repeatedly reciprocated between the projected position and the preset position or between the projected position and the withdrawn position, an additional considerable length of time is needed in the ejecting operation. This prolongs the time required to complete the ejecting operation, resulting in increased operational costs.
Moreover, an operator must set advancing speed, advancing pressure, withdrawing speed, etc., for the above-described reciprocation between advance and withdrawal, in addition to projecting speed, projecting pressure, withdrawing speed, etc., which regulate the advancing step and withdrawing step. Accordingly, the operation for setting these parameters is troublesome.
In still another conventional ejector device, the ejector pin 133 is withdrawn by a large stroke or a full stroke in the withdrawing step such that an ejector plate carrying the ejector pin 133 hits an unillustrated movable die mounting plate, thereby generating a shock, by which the molded product 131 is separated from the ejector pin 133. Also, in this ejector device, noise exceeding 90 dB is generated when the ejector plate hits the movable die mounting plate.